A new web-based and open data reduction application to determine Fe2+/Fe3+ ratios using the electron microprobe flank method.

The electron microprobe (EPMA) flank method can be used to determine in-situ the Fe2+/Fe3+ ratios in garnet [1], and potentially also in other minerals. It is a tool to reveal the redox history of rocks, especially when the garnet grain size or garnet homogeneity requires a microanalytical technique for Fe3+/Fe(tot) determination (e.g., [2,3,4]). The flank method is based on the accurate intensity measurement at two positions on the flanks of the FeLa and FeLb emission lines, and makes thus use of the systematic change of the intensities and wavelengths of the FeL lines with: (i) the iron oxidation state and (ii) the total iron content.  Data reduction of the obtained data so far required a complex combination of numerous Excel spreadsheets, and, for multiple linear regressions, the conversion to text files in combination with the Matlab clone Octave. This work-flow required a trained expert, and could quickly take up to even several days. 

We converted this process into a web-application with a simple and intuitive graphical user interface (GUI), with which the entire work-flow can be completed within minutes to hours. The web-application uses the 2-dimensional linear regression fit to determine Fe2+ from its dependency both on the intensity ratio FeLb/FeLa and the total Fe content, based on eq. 2 in [1].  The web-application allows the visual examination of all data using a large variety of plots for in-depth data inspection. Video tutorials embedded on the website explain not only how to use the website, or how the data reduction itself works, but also the flank as well as the EPMA method itself. Own data can be uploaded and reduced, and an available demo dataset allows training and exploring the web-application.

The entire web-application is realised using Python, Streamlit and a public GitHub repository. We will present what the flank method is, how it works using the EPMA, how the data reduction process works, and demonstrate how to use the web-application from the raw dataset to the final results.

[1] Höfer H. E. and Brey G. P. (2007) The iron oxidation state of garnet by electron microprobe: Its determination with the flank method combined with major-element analysis. Am Mineral 92, 873–885.

[2] Wang C, Tao R, Walters JB, Höfer HE, Zhang L (2022): Favorable P–T–ƒO2 conditions for abiotic CH4 production in subducted oceanic crusts: A comparison between CH4-bearing ultrahigh- and CO2-bearing high-pressure eclogite. Geochim. Cosmochim. Acta 336, 269-290

[3] Tang M, Lee C-TA, Costin G, Höfer HE (2019): Recycling reduced iron at the base of magmatic orogens. Earth and Planetary Science Letters 528, 115827. doi.org/10.1016/j.epsl.2019.115827

[4] Aulbach S, Höfer HE, Gerdes A (2019): High-Mg mantle eclogites from Koidu (West African craton): Neoproterozoic ultramafic melt metasomatism of subducted Archaean plateau-like oceanic crust. Journal of Petrology 60, 723-754